HESI A2 Biology Study Guide: Key Topics to Review

The HESI A2 Biology section draws on foundational concepts involving cells, genetics, metabolism, biological molecules, and cellular reproduction. Organizing these topics around how living systems obtain energy, store information, and maintain homeostasis can make the material easier to understand.

Prokaryotic and eukaryotic cells

Cells are the basic structural and functional units of life. They fall into two broad categories:

  • Prokaryotic cells: Lack a membrane-bound nucleus. Their DNA is located in a nucleoid region. Bacteria and archaea are prokaryotes.
  • Eukaryotic cells: Have a membrane-bound nucleus and other membrane-bound organelles. Animals, plants, fungi, and protists are eukaryotes.

Eukaryotes may be unicellular or multicellular. Yeast, for example, is a single-celled fungus.

Cell structures and organelles

Know the primary functions of common cell structures:

  • Cell membrane: Controls movement into and out of the cell
  • Nucleus: Stores most of the cell’s DNA
  • Ribosomes: Build proteins
  • Mitochondria: Produce most cellular ATP through aerobic respiration
  • Rough endoplasmic reticulum: Helps synthesize and process proteins
  • Smooth endoplasmic reticulum: Supports lipid synthesis and detoxification
  • Golgi apparatus: Modifies, sorts, and packages cellular products
  • Lysosomes: Contain enzymes that break down cellular material
  • Cytoskeleton: Supports cell shape, organization, and movement

Plant cells also contain a cell wall, chloroplasts, and a large central vacuole.

Cell membranes and transport

The cell membrane is primarily a phospholipid bilayer with embedded proteins. Its selective permeability helps the cell maintain a stable internal environment.

Review these transport processes:

  • Simple diffusion: Movement from higher to lower concentration without cellular energy
  • Facilitated diffusion: Passive movement through a membrane protein
  • Osmosis: Diffusion of water across a selectively permeable membrane
  • Active transport: Movement that requires cellular energy
  • Endocytosis: Movement of material into a cell through vesicles
  • Exocytosis: Release of material from a cell through vesicles

Biological molecules

Four major classes of biological molecules are:

  • Carbohydrates: Provide energy and structural support
  • Lipids: Store energy and form cell membranes
  • Proteins: Serve structural, transport, signaling, immune, and enzymatic functions
  • Nucleic acids: Store and transmit genetic information

Enzymes are proteins that lower the activation energy required for chemical reactions. Temperature, pH, and substrate concentration can affect enzyme activity.

Cellular respiration and photosynthesis

Cellular respiration converts energy stored in nutrients into ATP. Review the general roles of glycolysis, the citric acid cycle, and the electron transport chain.

Photosynthesis allows plants and other photosynthetic organisms to convert light energy into chemical energy. Chloroplasts are the primary site of photosynthesis in plant cells.

The products of photosynthesis support cellular respiration, while the products of cellular respiration can support photosynthesis.

DNA, RNA, and protein synthesis

DNA stores hereditary information. A gene is a segment of DNA that contributes to a functional product.

  • Replication: DNA is copied.
  • Transcription: Information in DNA is used to create RNA.
  • Translation: Ribosomes use messenger RNA instructions to assemble a protein.

Review the roles of messenger RNA, transfer RNA, ribosomal RNA, codons, and amino acids.

Mitosis and meiosis

Mitosis produces two genetically similar daughter cells and supports growth, tissue repair, and cell replacement.

Meiosis produces haploid gametes and introduces genetic variation. It includes two rounds of division and halves the chromosome number.

Be able to compare:

  • The purpose of each process
  • The number of divisions
  • The number and genetic similarity of the resulting cells
  • Diploid and haploid chromosome numbers

How to prepare for HESI A2 Biology

Build connections among topics instead of memorizing each term separately. For example, connect the nucleus to DNA, DNA to RNA, RNA to ribosomes, and ribosomes to protein production.

Use diagrams, retrieval practice, and practice questions across several weeks. When you miss a question, identify the underlying concept and review it again later.

Prepare for HESI A2 Biology with Pocket Prep. Detailed explanations can help you strengthen foundational biology concepts and learn from each answer.